Dental caries is a disease manifested by local demineralization of the hard tissues of the tooth crown induces by dental plaque. The demineralization process progresses from the outer enamel surface of the crown through the entire thickness of the enamel and into the dentine. Caries lesions of occlusal, buccal and lingual (palatinal) surfaces can be diagnosed by mechanical probing and/or visual inspection. On the other hand, small and medium size lesions of interproximal crown surfaces are hidden by the gingiva and adjacent teeth and have hitherto been identifiable only on radiographs. Although the use of bitewing radiographs is well accepted as an important adjunct in the diagnosis of proximal caries lesions, this method exhibits several weaknesses related to its relative insensitivity and user dependence in terms of technical performance and interpretation Waggoner W., F. Crall J. J. (1984) Quintessence International 11/1984: 1163-1173!. Furthermore, bitewing radiographs comprise a high proportion of x-rays taken in the dental office. This is in contrast with the current trends in safety standards which support every effort aimed at reducing the exposure to ionizing irradiation. In addition, an alternative technology for the detection of interproximal caries is expected to reduce the environmental pollution and cost associated with the use of x-ray technology.
The potential of ultrasonic technology for the detection of dental caries has been proposed in several instances. It has been shown that the hard tissues of the tooth crown, in particular the outer enamel layer, are highly uniform in their sonic properties among different teeth and individuals Ng S. Y., Payne P. A., Cartledge N. A., Ferguson M. W., (1989), Arch. Oral Biol. 34: 341-345; Barber F. E., Lees S., Lobene R. R., (1969), Arch. Oral Biol. 14: 745-760!. Using a longitudinal ultrasonic irradiation, a specific profile of ultrasonic echoes is obtained from the enamel surface, dentinoenamel junction and pulpodentinal junction. Changes in this profile have been described in instances of demineralization lesions indicating a substantial difference in the sonic conductivity between sound and demineralized enamel Ng S. Y., Ferguson M. W. J., Payne P. A., Slater P., (1988), J. Dent 16:201-209; PCT no. WO 95/04506!. These changes result from conversion of the intact enamel to the water rich demineralized material. However, the detection of these lesions is dependent on a direct contact between the ultrasonic probe and the demineralized enamel; such contact cannot be formed in interproximal sites. In addition, echo profiles to longitudinal waves obtained from sound and demineralized tooth crowns are complicated and can be analyzed only by using complex systems.
The present invention relates to a revolutionary approach regarding the use of ultrasonic technology for the detection of dental caries. Rather than transmitting longitudinal ultrasonic waves into the tooth, as in the prior art, the present invention relates to an ultrasonic device that imparts surface ultrasonic waves onto the enamel surface, which then migrate along same. Surface or Rayleigh waves are well known Cook, E. G., Van Valkenburg H. E., (1954) ASTM-Bull 84!. These waves are generated by an ultrasonic probe in which the longitudinal axis of the piezoelectric crystal, herein defined as an imaginary line passing through the geometrical centre of the crystal surface and perpendicular thereto, lies at an angle substantially smaller than 90.degree. to the tested surface, by virtue of an appropriately designed wedge-like interface. Surface waves migrate uninterruptedly on smooth, flat or curved, contours. Sharp angles, interferences and sonic interfaces present on the surface produce distinct echoes Krautkraimer J., Krautkramer H., Ultrasonic Testing of Materials, (1969) Springer-Verlag Berlin Heidelberg New York; pp. 257-271!. The amplitude and shape of these echoes is dependent on the geometry of such interferences. Thus, the interface between a caries lesion and intact enamel may be identified by an echo or reflection of surface ultrasonic waves produced thereat. Since said reflected surface ultrasonic waves have an amplitude substantially greater than the general background level, the profiles of said waves are relatively simple to analyse. Furthermore, since the ultrasonic device of the present invention does not require to be placed directly onto the zone of interest on the tooth surface, it is particularly useful for the detection of caries lesions in areas such as the interproximal site, hitherto inaccessible with ultrasound devices of the prior art.